1. Field of the Invention
The present invention relates to a film substrate that is mounted in an image display flat panel or the like and, more particularly, to a conductor-wiring structure for the film substrate and the fabrication method thereof.
2. Description of the Related Art
One form of semiconductor package that is mounted in an image display flat panel or the like is a tape carrier package, which is mainly used as a driver for driving purposes. Among such tape carrier packages, a tape carrier package in which holes in the film base material are not formed in semiconductor element mount portions of the film base material in particular is known as COF (Chip On Film; a mounting structure that uses a thin film for the substrate and in which the chip and circuit on the film substrate are electrically connected by means of bumps).
A commonly known film substrate with the COF structure is disclosed in Japanese Patent Application Laid Open No. 2002-223052, for example. The film substrate disclosed in this publication is a flexible substrate that is connected to the substrate of a liquid-crystal display panel. Further, the film substrate comprises a film base material on which active elements such as semiconductor chips, passive elements such as resistors and condensers, and mechanical parts, and so forth, are mounted, wherein a plurality of terminals formed from copper thin film exist in the vicinity of a terminal portion that is connected to the liquid-crystal display panel on the film base material.
The film substrate and the substrate of the liquid crystal display panel are connected by means of an ACF (Anisotropic Conductive Film). The ACF is formed in a state in which conductive particles are distributed scattered within an insulating resin. The film substrate and the substrate of the liquid crystal display panel are bonded together by means of the insulating resin by subjecting the film substrate and the substrate of the liquid crystal display panel to thermo-compression bonding with the ACF interposed therebetween and, thus, each of the terminals of the film substrate and the electrodes or connecting terminals facing the substrate of the liquid crystal display panel are conductively connected by means of conductive particles.
Furthermore, each of the terminals of the film substrate and the elements and parts that are mounted on the film substrate are connected by means of conductive wiring that is formed on the film base material. A method that forms the conductive wiring by etching a metal foil on the film base material and a method that forms the conductive wiring directly on the film base material by means of metal plating are known.
Further, the increasing number of output terminals of semiconductor elements that are mounted on film substrates and the reductions in the size of semiconductor elements as a result of the increased detail of liquid crystal display panels in recent years has been accompanied by the trend in the annually decreasing pitch of semiconductor-element mount portions of film substrates and of parts with conductor wiring in the vicinity of the mount portions (conductor wiring portions) and, hence, there is a desire for a reduction in the pitch of conductor wiring portions. In order to implement a reduction in the pitch of conductor wiring portions, a decrease in the thickness of conductor wiring portions is advantageous for the two above methods, that is, for both the method that forms conductor wiring portions by etching and metal foil on the film base material and the method that forms the conductor wiring portion directly on the film base material by means of metal plating. However, when the thickness of a conductor wiring portion is reduced, the gap between the film substrate and the substrate of the liquid crystal display panel is then smaller at the join with the substrate of the liquid crystal display panel and the density of conductive particles in the ACF increases. As a result, short circuit failure between electrodes of the substrate of the liquid crystal display panel, between connecting terminals, or between conductor wiring of the film substrate readily occurs.
Furthermore, the film substrate is sometimes bent and connected to the substrate of the liquid crystal display panel by making use of the flexibility the film substrate. Here, when the thickness of the part in which the film substrate is bent (bent portions) is large, the radius of curvature of the conductor wiring portion is large and the conductor wiring portion readily breaks.